Polar Bear Swim Day

What the day is

Polar Bear Swim Day is observed on January 1. Organized New Year plunges run in cold-water cities worldwide, several of them a century old, including long-running events in Vancouver and at Coney Island, a Dutch new year dive that draws tens of thousands, and Scotland's version in the Firth of Forth.

Origin and background

The physiology is the part worth knowing, because the popular understanding of the risk is wrong.

People assume the danger is hypothermia. It is not, at least not in the first instance. Dropping into water below roughly fifteen degrees Celsius triggers the cold shock response: an involuntary gasp, followed by a minute or two of hyperventilation you cannot fully control, along with a sharp rise in heart rate and blood pressure. If that gasp happens underwater, it is fatal, and this is what kills most people who die shortly after entering cold water. Hypothermia takes much longer, generally half an hour or more, which is why survival advice emphasizes surviving the first minute.

The cardiovascular jolt is also a genuine risk for anyone with an undiagnosed or known heart condition, and it is the reason these events carry medical cover.

The other effect people underestimate is afterdrop. Core temperature keeps falling for a while after you are out of the water, as cold blood from the limbs returns to the center, so the coldest moment comes later than expected. Alcohol makes all of this worse by opening peripheral blood vessels, and the traditional warming drink is precisely the wrong idea.

On the wellness claims now attached to cold plunging, the evidence for dramatic benefit is thin and mixed, while the risks above are well established.

How to take part

• Go to an organized event with supervision rather than plunging somewhere alone.
• Walk in rather than jumping or diving, and keep your head up.
• Breathe out slowly and deliberately through the first minute. That is the whole technique.
• Stay in for a very short time. Minutes, not a swim.
• Have dry clothes and something warm ready, and skip the alcohol until you have rewarmed.
• Do not go in at all if you have a heart condition without asking a doctor first.